Contrast ratio (CR) is one of the most important attributes considered in determining the quality of both normally white (NW) and normally black (NB) LCDs. The contrast ratio (CR) in a normally white display is determined in low ambient conditions by dividing the "off-state" light transmission (high intensity white light) by the "on-state" or darkened transmitted intensity. For example, if the "off-state" transmission is 200 fL at a particular viewing angle and the "on-state" transmission is 5 fL at the same viewing angle, then the display's contrast ratio at that particular viewing angle is 40 (or 40:1) for the particular driving voltages utilized.
Accordingly, in normally white LCDs, a significant factor adversely limiting contrast ratio is the amount of light which leaks through the display in the darkened or "on-state." In a similar manner, in normally black displays, a significant factor limiting the contrast ratio achievable is the amount of light which leaks through the display in the darkened or "off-state." The higher and more uniform the contrast ratio of a particular display over a wide range of viewing angles, the better the LCD in most applications.
Normally black (NB) twisted nematic displays typically have better contrast ratio contour curves or characteristics than do their counterpart NW displays (i.e. the NB image can often be seen better at large or wide viewing angles). However, NB displays are optically different than NW displays and are much more difficult to manufacture due to their high dependence on the cell gap or thickness "d" of the liquid crystal layer as well as on the temperature of the liquid crystal (LC) material itself. Accordingly, a long-felt need in the art has been the ability to construct a normally white (NW) display with high contrast ratios over a large range of viewing angles, rather than having to resort to the more difficult and expensive to manufacture NB displays in order to achieve these characteristics.
What is often needed in NW LCDs is an optical compensating or retarding element(s), i.e. retardation film(s), which introduces a phase delay that restores the original polarization state of the light, thus allowing the light to be substantially blocked by the output polarizer (analyzer) in the "on-state." Optical compensating elements or retarders are known in the art and are disclosed, for example, in U.S. Pat. Nos. 5,184,236; 5,189,538; 5,406,396; 4,889,412; 5,344,916; 5,196,953; 5,138,474; and 5,071,997.
The disclosures of U.S. Ser. No. 08/559,275; and U.S. Pat. Nos. 5,570,214 and 5,576,861 (all incorporated herein by reference) in their respective "Background" sections illustrate and discuss contrast ratio, and driving voltage versus intensity (fL) graphs of prior art NW displays which are less than desirable. Many prior art NW LCD viewing characteristics are problematic in that, for example, their contrast ratios are limited horizontally and/or vertically (and are often non-symmetric), and their gray level performance lacks consistency.
Gray level performance, and the corresponding amount of inversion, are also important in determining the quality of an LCD. Conventional active matrix liquid crystal displays (AMLCDs) typically utilize anywhere from about 8 to 64 different driving voltages. These different driving voltages are generally referred to as "gray level" voltages. The intensity of light transmitted through the pixel(s) or display depends upon the driving voltage utilized. Accordingly, conventional gray level voltages are used to generate dissimilar shades of color so as to create different colors and images when, for example, the shades are mixed with one another.
Preferably, the higher the driving voltage in a normally white display, the lower the intensity (fL) of light transmitted therethrough. The opposite is true in NB displays. Thus, by utilizing multiple gray level driving voltages, one can manipulate either a NW or NB LCD to emit desired intensities and shades of light/color. A gray level voltage V.sub.ON is generally known as any driving voltage greater than V.sub.th (threshold voltage) up to about 3.0 to 6.5 volts, although gray level voltages may be as low as 2.0 in certain applications.
Gray level intensity in an LCD is dependent upon the display's driving voltage. It is desirable in NW displays to have an intensity versus driving voltage curve for as many viewing angles as possible wherein the intensity of light emitted from the display or pixel continually decreases as the driving voltage increases. Such desired gray level curves across a wide range of view allows the intensity of light reaching viewers at different viewing angles to be easily and consistently controlled. It is also desirable that all such curves have as close to the same slope as possible.
U.S. Pat. No. 5,583,679 (the disclosure of which is incorporated herein by reference) discloses an LCD including an optical compensating sheet that includes a discotic structure and negative birefringence, with the discotic structure unit having an inclined plane. Unfortunately, the contrast ratios and inversion characteristics resulting from displays of the '679 patent have been found by the instant inventors to be less than desirable. Certain embodiments of the instant invention described herein exhibit surprisingly improved results with respect to contrast ratio and/or inversion as compared to the '679 patent.
It is apparent from the above that there exists a need in the art for a normally white TN liquid crystal display (LCD) wherein the viewing zone of the display has high contrast ratios and/or little or no inversion over a wide range of viewing angles. Furthermore, there exists a need in the art for improved contrast and reduced inversion in the same viewing zone (e.g. in the upper vertical viewing zone principally utilized by pilots of aircraft in avionic applications).
The term "rear" when used herein as it is used to describe substrates, polarizers, electrodes, buffing films or zones, retarders, and orientation films means that the described element is on the backlight side of the liquid crystal material, or in other words, on the side of the LC material opposite the viewer.
The term "front" when used herein but only as it is used to describe substrates, polarizers, retarders, electrodes, buffing films or zones and orientation films means that the described element is located on the viewer side of the liquid crystal material.
Unless otherwise specified, the actual LCDs and light valves made and/or tested herein included a liquid crystal material with a birefringent value (.DELTA.n) of 0.0854 at room temperature, Model No. ZLI-4718 obtained from Merck.
Unless otherwise specified, the term "retardation value" as used herein for uniaxial retarders means "d .DELTA.n" of the retardation film or plate, where "d" is the film or plate thickness and ".DELTA.n" is the film birefringence (i.e. difference in certain indices of refraction).
The term "interior" when used herein to describe a surface or side of an element (or an element itself), means that closest to the nematic liquid crystal (LC) material. The term "exterior" means the side farthest from the nematic liquid crystal layer.
The term "light valve" as used herein means a liquid crystal display including a rear linear polarizer, a rear transparent substrate, a rear continuous pixel electrode, a rear orientation film, an LC layer, a front orientation film, a front continuous pixel electrode, a front substrate, and a front linear polarizer (i.e. without the presence of color filters and/or active matrix driving circuitry such as TFTs). Such a light valve may also include retardation film(s) disposed on either side of the LC layer as described with respect to each example and/or embodiment herein. In other words, a "light valve" (LV) may be referred to as one giant pixel without segmented pixel electrodes.
For all circular grid contrast ratio graphs herein, "EZContrast" equipment available from Eldim of Caen, France (ID #204F) was used to develop these graphs. This equipment includes a system for measuring Luminance and Contrast versus viewing angle (incident (polar) and azimuth angle), utilizing 14 bits A/D conversion to give luminance measurements from 1/10 to 8,000 cd/m.sup.2, with an accuracy of 3% and a fidelity of 1%. A temperature regulated CCD sensor with a photopic response (and specially designed lenses) are part of this commercially available Eldim system and corresponding software. The measurement device of this Eldim system includes a specially designed large viewing angle lens system having a numerical aperture of 0.86. The Eldim software is Windows.TM. 3.1 based, running on any 486 and above PC, supporting DDE interface with other programs.
All measured real data herein, in the Examples, included the non-uniform characteristics of the backlight over a range of angles. Backlights are more intense at normal than at wide angles.